EXPLAIN SELECT * FROM reservations WHERE no_of_guests>3;
+------+-------------+--------------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table        | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+--------------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | reservations | ALL  | NULL          | NULL | NULL    | NULL |    6 | Using where |
+------+-------------+--------------+------+---------------+------+---------+------+------+-------------+

CREATE INDEX reservations_idx ON reservations(no_of_guests);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0


EXPLAIN SELECT * FROM reservations WHERE no_of_guests>3;
+------+-------------+--------------+-------+------------------+------------------+---------+------+------+-----------------------+
| id   | select_type | table        | type  | possible_keys    | key              | key_len | ref  | rows | Extra                 |
+------+-------------+--------------+-------+------------------+------------------+---------+------+------+-----------------------+
|    1 | SIMPLE      | reservations | range | reservations_idx | reservations_idx | 5       | NULL |    1 | Using index condition |
+------+-------------+--------------+-------+------------------+------------------+---------+------+------+-----------------------+